1. Field of the Invention
The present invention relates to a steering system that changes a transfer ratio of a steering angle of a steering wheel to a turning angle of a turning wheel by means of a variable transfer ratio mechanism and generates auxiliary power during steering by means of an electric power steering unit.
2. Description of Related Art
Heretofore, a steering system having a variable transfer ratio mechanism and an electric power steering unit has been proposed. The variable transfer ratio mechanism enables a driver to largely turn a turning wheel with less steering operation by setting a transfer ratio (=angle of steering wheel/angle of turning wheel) to be small (by quickening the ratio) in a low vehicle speed range during parking and the like, so that the mechanism can reduce a burden of operation of the driver and improve convenience in driving.
The variable transfer ratio mechanism also enables yaw rate response characteristics and vehicle traveling stability to be improved by setting the transfer ratio to be large (by slowing the ratio) in a high vehicle speed range because the turning wheel is turned less even if the driver largely steers the steering wheel. The electric power steering unit also enables the driver to turn the turning wheel with less steering operation because the unit generates steering auxiliary power.
If the driver tries to turn the steering wheel quickly when the ratio is quickened in the low vehicle speed range, the steering wheel of the steering system provided with such variable transfer ratio mechanism and electric power steering unit may become tough to turn because deficiency of thrust force occurs due to an influence of counter electromotive voltage of an EPS (Electric Power Steering) motor, i.e., a power source of the electric power steering unit, even through the electric power steering unit tries to generate the auxiliary power so as to be able to turn the turning wheel largely and quickly.
Then, there has been proposed a method of avoiding the steering wheel from becoming tough to turn by changing a target transfer ratio, i.e., a target value of the transfer ratio, in real time so that the transfer ratio is changed in a direction of slowing down the ratio in such a case as disclosed in Japanese Patent Application Laid-open No. 2000-344120 for example.
However, the prior art method was considered to cause such phenomena that steering reactive force fluctuates as if it undulates with respect to such steering operations of quick-slow-quick for example or that the steering reactive force increases in the beginning of the steering operation and the steering operation is prone to be stuck (caught) when the driver starts to turn the steering wheel abruptly with large steering speed.
The driver feels a sense of discomfort in operating the steering wheel when such phenomena occur.
Still more, when the driver turns the steering wheel with large steering speed and when an actual transfer ratio cannot quickly follow up the target transfer ratio due to output torque of the transfer ratio variable motor used in the variable transfer ratio mechanism, the driver may possibly feel a sense of discomfort in operating the steering wheel as if the turn of the turning wheel continues in a previous steering direction even after finishing the operation of the steering wheel.
Accordingly, the present invention seeks to provide a steering system that enables the driver to operate a steering wheel comfortably even if the driver turns the steering wheel quickly when the transfer ratio is quickened in the low vehicle speed range.